1. Field of the Invention
The present invention relates to a battery management system (BMS) and a battery management method, and more particularly, to a battery management system and a battery management method capable of managing charging and discharging of each of a plurality of battery cells according to individual statuses of the cells.
2. Description of the Prior Art
Generally, a battery is required to undergo a sorting process before packaging and leaving a manufacturing plant. This is due to old and new batteries (i.e. batteries having different statuses) cannot be mixed in use. A mixed use of batteries leads to an imbalanced distribution of power, where weaker batteries are forced to output a higher power and depletes faster than newer batteries.
Sorting the batteries into groups of similar statuses would help alleviate this imbalance and allow cells to deplete at similar rates. Older batteries are usually characterized by having higher internal resistances. When connected serially in a current loop with other batteries, older batteries having higher internal resistances tend to output a relatively higher power (P=I2R, wherein a current I is same for each element in the current loop, and R is the internal resistance of a battery cell). In contrast, healthier cells having lower internal resistances are required to output lower power. Also, a higher output power means a higher operating temperature, and thus a danger of thermal runaway. Both factors speed up the widening of status differences between the cells, as well as accelerate the deterioration of weaker batteries toward depletion, at which point the entire current loop would be cut off. Smaller devices such as notebook computers utilize smaller battery packs (e.g. 6-cell); however, for higher-power applications (e.g. electric vehicles) which often have battery packs with hundreds of cells, it can be extremely costly to have to replace a whole battery pack when only one cell is dead.
However, this sorting can be difficult, as batteries usually exhibit only subtle status differences before leaving the plant. It is not only until certain amount of usage that the imbalances and discrepancies between batteries begin to emerge (a non-linear and accelerating process, as mentioned above). This difficulty in sorting is exacerbated by the fact that in applications that require outputting large instantaneous currents such as electric vehicles (e.g. for cruising sloped environments), batteries are required to have extremely low internal resistances and a high C rate (a.k.a. hourly rate), for example, LiMPO4 (Lithium iron phosphate) batteries. This means that initial status differences between batteries are even more subtle, rendering the sorting process impossible. Also, electric vehicles may utilize hundreds of cells, and attempting to obtain hundreds of cells having similar statuses during the sorting process is also a major challenge.
Thus, there is a need to improve over the prior art.